A rotary electrohydraulic servovalve typically has an output stage having a multiple-lobed spool member rotatably mounted within a body to control the flows of pressurized fluid through variable-area metering ports or orifices to and from the opposed chambers of a fluid-powered load. Preferably, the metering ports are rectangularly-shaped and the cooperating lobe edges are straight and parallel to the axis of spool rotation so that the orifices will also be rectangular, thereby causing the valve to have a constant flow gain (i.e., so that flow through the valve will be substantially proportional to angular displacement of the spool member from a null position).
It is well known that the flow jet through such square-edged metering orifices will inherently be at an angle of about 69.degree. to the circumferential plane of the metering port opening. This flow angle will create a reaction force on the spool having both tangential and radial components. The radial component will not have any appreciable effect on the spool member. However, the tangential component will cause an orifice-reducing reaction force. These flow reaction forces are sometimes called "Bernoulli" forces, and are illustrated and described in Blackburn et al., Fluid Power Control, The M.I.T. Press (1960) [.sctn.10.3 et seq.].
While the presence of such Bernoulli forces has been known, their existence has become particularly problematic in direct-drive rotary servovalves in which a spool member is rotated directly by a limited-force torque motor. Such flow forces may be reduced by application of the same techniques which have been employed on linear spool valves. (See, e.g., Blackburn et al., supra, and R. N. Clark, "Compensation of Steady-State Flow Forces in Spool-Type Hydraulic Valves", A.S.M.E. Paper No. 56-A-121 (1957).) This paper discusses the existence of such flow forces, the trend toward smaller and smaller valve drivers, and illustrates three possible ways of compensating the effect of such forces. These compensation schemes include: (1) the provision of radial-hole orifices, (2) the provision of "recirculation lands" downstream of the flow-metering orifices, and (3) pressure-drop compensation. Such downstream "recirculation lands" are also shown in Blackburn et al., supra, at section 10.321, although Blackburn appears to denominate them as "negative-force ports" rather than "recirculation lands".
Ohrendorf et al. (U.S. Pat. No. 4,479,5 12) discloses a type of high-flow combined rotary and linear servovalve that recognizes the existence of Bernoulli-type flow forces that exert orifice-closing reaction forces on the spool member, and suggests applying the methods discussed by Clark (see, e.g., col. 2, line 10 et seq. and col. 6, line 43 et seq.). However, Ohrendorf does not appear to eliminate substantially all of the flow forces acting on the spool member.
Applicant's earlier U.S. Pat. No. 4,794,845 discloses a type of high-flow direct-drive rotary servovalve having a spool member rotatably mounted within a sleeve-like body. While this patent posited that careful control of the angle of entry of the flow through the metering ports (i.e., between the pressure source and the load only) established by certain slots could minimize or nearly eliminate the Bernoulli flow forces (see, e.g., col. 5, line 66 et seq.), the referenced slots or surfaces are, in fact, located downstream of the orifices in question and thus do not affect the flow entry angle. It has since been determined that the observed reduction of flow forces was due to the fortuitous configuration of the upstream spool passage wall 32, as shown in FIG. 2 and described below. The operative fluid flow phenomenon has been observed in linear displacement servovalves which were modified to reduce the effect of viscosity on the metered flow by relieving the surfaces immediately upstream of metering orifices, causing them to appear as "sharp-edged" rather than as "short tubes".
While these references do recognize the fact that Bernoulli-type flow forces act on the spool in such a way as to tend to close the flow-metering orifice, they do not appear to have contemplated the desirability of preventing such forces from occurring in the first instance. Because of the continuing tendency toward smaller and more-compact valve drivers, it is desirable to substantially eliminate such flow forces, rather than to overpower them or compensate for their existence.